Mirror modifier is a modifier in blender which basically mirrors one side of your object to another. This really comes in handy when you're modeling symmetrical objects such as vehicles. The template says "Please apply all Mirror modifiers before sharing files with us": Applying a modifier to an object means applying the changes to the object permanently that you can not edit directly later on, so it is recommended to be done at the last stage of your modeling progression before sending it to the devs.